In processing video signals, it is known to be particularly advantageous to separate the luminance and chrominance components from composite video signal with comb filters. The reason being that the separated luminance component is of full bandwidth, e.g., 4.2 MHz for NTSC signal, and cross components are substantially eliminated from both of the separated luminance and chrominance components. Typical intraframe comb filters for NTSC video signals include circuitry for combining signals which are displaced in time by an odd integral number of horizontal line periods. Because the phase of the chrominance subcarrier changes by exactly 180.degree. from line to line, additively combining NTSC video signals displaced by one line interval produces a signal in which the luminance components from the two lines constructively combine and the chrominance component is cancelled. Conversely, if the signals are subtractively combined the luminance component is cancelled while the chrominance components from the two lines constructively combine. The vertical resolution of the luminance component is compromised but is acceptable.
PAL signals on the other hand have a chrominance subcarrier which exhibits a 180.degree. phase change every two horizontal line periods. Therefore, typical intraframe PAL comb filters combine video signals which are displaced in time by two horizontal intervals. The PAL comb filter function is basically the same as for the NTSC comb filter however, since the signals that are combined are spatially displaced by two lines, the vertical resolution of the PAL luminance component is significantly impaired, and tends towards not being acceptable.
Yoshimitsu Nakajima et al in an article entitled "Improvement of Picture Quality for NTSC and PAL Systems by Digital Signal Processing", IEEE Transactions on Consumer Electronics, Vol. CE-31, No. 4, Nov. 1989, pp. 642-654 describe adaptive comb filters which combine samples from three successive lines of video signal. In this system, the NTSC embodiment combines samples which are vertically aligned, similar to the aforedescribed typical NTSC and PAL comb filters. The PAL embodiment however combines diagonally aligned samples from adjacent lines. This tends to preserve the vertical resolution of the luminance component, reduces memory requirements for delaying the video signals but tends to have an adverse affect on images containing vertical lines.
Simple comb filters tend to introduce cross components in the separated luminance and chrominance signals where differences exist in the composite signal line-to-line. It is known, however, that such cross components may be significantly reduced by the process of adaptive comb filtering. Examples of adaptive comb filters may be found in U.S. Pat. No. 4,786,963 issued to McNeely et al and U.S. Pat. No. 4,803,547 issued to Stratton. In adaptive systems, signals from a plurality of adjacent lines are provided. These signals are compared to determine which signals, when combined to provide a comb filter output, will tend to produce the most desirable signal.
In order to realize economies of scale in the manufacture of video signal processing components it is advantageous to produce multistandard processing elements which are useful for, e.g., NTSC and PAL signal systems. To this end it is desirable to have a multistandard comb filter. Shinichi Nakagawa et al. in U.S. Pat. No. 4,727,415 describe an adaptive multistandard comb filter system. In this system, for the NTSC mode, the comb filiter adaptively combines composite video samples from adjacent lines of video signal, to produce separated luminance and chrominance components. In the PAL mode, the comb filter adaptively combines composite video samples displaced by two horizontal lines to produce separate luminance and chrominance components. As such, in the PAL mode, there is a loss of vertical resolution.